Noting that sound is created by the fluctuation of air pressure caused by the movement and/or vibration of a given object, the impact of loudspeaker size on loudspeaker performance is clear. It is also apparent that with the trend toward slim/thin form factors in mobile computing platforms, such as smart phones and other mobile/wearable devices, it is increasingly difficult for device manufacturers to design loudspeaker solutions. The size of loudspeakers that fit inside modern mobile devices are simply too small to move large volumes of air. As a result, loudspeaker performance in mobile devices has stagnated or even degraded in recent years.
FIG. 1A illustrates a mobile computing device assembly 101, which includes a chassis 102 and a pair of micro-speakers 105 affixed to chassis 102. A display, microprocessor and other integrated circuitry, as well as a battery may be installed into assembly 101 in any conventional manner. During device operation, loudspeaker membrane movement within micro-speaker 105 is to generate audio output 115. Micro-speaker(s) 105 is driven to provide monaural or stereo output. Micro-speakers 105 are typically tuned to a resonance frequency that enables greater loudness or sound pressures. FIG. 1A further illustrates a conventional micro-speaker design employing a back-side cavity 110, also known as an “air spring” which reinforces the micro-speaker sensitivity (loudness). The back-side cavity encloses a back-side of the loudspeaker membrane opposite a front cavity through which the sound is outlet. The size of the back-side cavity is a function of the micro-speaker design and desired frequency response. Often, a third, or more, of a micro-speaker volume is occupied by the back-side cavity. To accommodate the micro-speaker with back-side cavity, a device design may either increase chassis length (e.g., y-dimension) or chassis thickness (e.g., z-dimension).
FIG. 1B illustrates a frequency response 120 typical of a mobile computing device employing the micro-speaker design illustrated in FIG. 1A. Sensitivity at a distance from the source of 10 cm is shown for a 2V. (500 mW) drive signal. Frequency response 120 has a peak at a resonance frequency of around 1 kHz associated with the back-side cavity, and another peak at a resonance frequency of around 4-5 kHz associated with front-side acoustics. While sound pressure is significant at the micro-speaker resonance frequencies, perceived loudness rolls off rapidly within a few hundred Hertz on either side of the resonance frequencies for a narrow band output perceived by the human auditory system as “thin” sounding. Such a perception impedes a user's enjoyment of hands-free calls, audio/video (A/V) playback, and gaming.
To facilitate a continued shrinking of mobile device platform dimensions, loudspeaker performance may continue to decline until loudspeaker integration is discontinued, at which point device users will be required to rely exclusively on ear buds, or other auxiliary audio speakers. Loudspeaker solutions that can augment existing micro-speaker designs to achieve greater audio performance, or replace existing micro-speaker designs to enable more flexibility in device form factor and scalability are therefore advantageous.